A. Field of the Invention
This invention refers to a composition and to a method for the commercial production of a privacy glass, of neutral gray color, having a light transmission less than 20% and an infrared radiation transmission less than 14% and especially an ultraviolet transmission less than 12% for use in the construction industry and mainly in the automotive industry.
B. Description of the Related Art
Several patents have been developed for obtaining gray glass, for "privacy" purposes, having low light transmission and low infrared and ultraviolet radiation transmission.
The glasses disclosed in almost all the prior patents referring to a type of neutral gray glass for privacy purposes, are based on three basic components: iron oxide, cobalt oxide and selenium, using different proportions and, together with the typical formulation of a soda-lime-silica glass, constitute the basic composition of the glass.
That is the case of the glasses of the U.S. Pat. No. 4,873,206 of James Jones, issued Oct. 10 of 1989, the U.S. Pat. No. 5,393,593 of Gulotta et al, issued Feb. 28 of 1995, and the U.S. Pat. No. 5,278,108 of Cheng et al, issued Jan. 11 of 1994, in which these components are used without any additional component.
Some other glasses disclosed in other patents, such as those mentioned in the following paragraphs use, besides the three mentioned components, different metallic elements conferring the characteristics to the final product, that allow them to be classified as privacy glasses, in order to be used in the construction and automotive industries.
The U.S. Pat. No. 5,308,805 of Baker et al, issued May 4 of 1994, describes a neutral gray glass having a low transmission, in which one of the claimed components is the nickel oxide in proportions of 100 to 500 ppm.
In the past, the heat absorbing gray glasses containing nickel in their structure, frequently presented inclusions of nickel in the form of sulfide which, during the melting of the glass were formed until appearing as small invisible particles which were impossible to distinguish in viewing an already formed glass. These inclusions of nickel sulfide are due to their high coefficient of thermal expansion that can cause thermal stresses sufficient to fracture a glass plate. This is a singular problem when the glass pieces are subjected to a thermal treatment as the tempering, in which the presence of the nickel sulfide cause an excessive percentage of breakdown pieces during or as a consequence of the tempering process.
An additional disadvantage of the glasses containing nickel is the change of color that they undergo after the thermal process, such as for example, after tempering.
U.S. Pat. No. 5,023,210 of Krumwide et al, issued Jun. 11 of 1991, discloses a low transmission neutral gray glass composition that does not includes nickel. In order to achieve similar characteristics to that of a neutral gray glass, Krumwide uses chromium oxide in amounts of 220 to 500 ppm as Cr.sub.2 O.sub.3, in its composition which, in these proportions, produces a gray tone and adjusts the levels of selenium and cobalt oxide in order to make it a neutral tone. However, in previous references it is mentioned a preference of not using these compounds because of the problems presented by the difficulty of melting the chromium compounds (U.S. Pat. No. 4,837,206), and additionally because these have difficulties in order to discard the solid materials containing said compounds. Also, in the U.S. Pat. No. 5,308,805, there is mentioned the inconvenience of the chromium oxide used like coloring agent, since it requires the use of additional operations and apparatuses to the conventional ones into the melting furnaces in order to reach the necessary conditions to produce the desired glasses.
The U.S. Pat. No. 5,346,867 of Jones et al, issued Sep. 13 of 1994, discloses a heat absorbing glass composition having a neutral gray color, which uses manganese and titanium oxide in order to increase the retention of the selenium (that is a high cost component), during the production process. Although from previous references (U.S. Pat. No. 4,873,206), it was known that the use of the manganese has a tendency to form a yellowish-brown coloration when it is exposed to the ultraviolet radiation making it difficult to maintain the uniformity of the product, and the use of the titanium causes a yellowish coloration when the glass enters in contact with the liquid tin of the float process. This is what makes undesirable these two aspects during the production of the glass because it makes critical the control of color in order to obtain the desired tone during the manufacture. Jones et al mentioned in their patent '867, that the process of solarization is a phenomenon associated with the change of Fe.sup.3+ to Fe.sup.2+ which cause an undesirable change in the color, mentioning that they found that this does not occur in the disclosed glass and additionally the use of the titanium oxide is incorporated into the glass in order to obtain the desired range of dominant wave length, as well to reduce the transmission of ultraviolet radiation.
On the other hand, it is well known by the persons skilled in the art, that the addition or substitution of one or more colorants for other colorants, or the change in the relative proportional amount in the glass composition, affects not only the color of the product, as for example the dominant wave length of the color or the excitation purity, but also the luminous transmission, the heat absorption and additional properties such as the transmission of ultraviolet and infrared radiation.
It has been known that copper played an important role in the production of colored glass, ceramics and pigments. It has been recognized, for example, the coloration of the Persian ceramic for their tonality conferred by the copper. Of special interest for ceramic artists are the turquoise blue and especially the Egyptian and Persian blue dark (Woldemar A. Weil; Colored Glasses, Society of Glass Technology, Great Britain, p. 154-167, 1976).
Copper has been used in the glass compositions, not only in those of soda-lime-silica type, but also in others such as those containing, for example, borosilicate. Therefore, the developed color depends on the base of the glass, on its concentration and on its oxidation state.
For the case of the mentioned glass as a base, the copper in the form of the oxide imparts a blue coloration of a greenish tone, specifically turquoise, however, in the glass, the copper can be in its monovalent state, which does not impart color. So, the blue greenish coloration depends not only on the amount of copper present, but on the ionic balance between the cuprous and cupric states. The maximum absorption of the copper oxide is in a band centered at 780 .eta.m and a maximum weak secondary peak is present at the 450 .eta.m, which disappears at high soda content (around 40% weight). (C. R. Bamford, Colour Generation and Control in Glass, Glass Science and Technology, Elsevier Scientific Publishing Company, p. 48-50, Amsterdam, 1977).
In the production of red ruby glass, a mixture containing copper oxide together with any reducer agent (SnO is commonly used), is melted in reduced conditions. The initial mixture shows the blue characteristic color of the copper II, but as soon as the melting begins, changes the color to a pale straw yellow which takes place during this stage. Due to a thermal treatment to a temperature between the annealing point and the softening point, the ruby red color is developed. If, during the melt, the reduction state is carried out beyond a critical stage, the color changes to brown and appears opaque or "turned off". On the other hand, if the copper is insufficiently reduced, some traces of blue color are kept and the ruby red color is not developed (Amal Paul, Chemistry of Glasses, Chapman and Hall, p. 264-270, London, 1982).
U.S. Pat. No. 2,922,720 of Parks et al, issued Jun. 20, 1957, discloses the use of the copper in the glass as: " . . . The copper has been used as coloring agent for glass upon developing a coloration ruby red, but in order to obtain the color in a open melt furnace, it has been necessary the use of cyanogens as reducer agent . . . ", additionally mentions the effect of the copper in the coloration of the glass, as due to the colloidal suspension of particles of metallic copper in the glass, and by analogy it is believed that a particle size produces the ruby red colors, depending on the intensity of the color of the copper concentration. For smaller particle sizes, the effect of color is null.
The glass of the present invention, avoids the use of some metals that the above mentioned patent disclose as very important, this is the case of the nickel, the chromium and the manganese.
In the glass of the present invention, the copper oxide (CuO) is used additionally to the use of iron oxide, cobalt oxide and selenium, as an alternative for obtaining of a lightly greenish gray tonality in the region of neutral color frequently used in the automotive and construction industry. Additionally providing a reduction in the ultraviolet radiation transmission and a reduction in the near infrared region for the bands of absorption around the 800 nanometers which help to reduce the infrared solar transmission.
So that, in the present invention, a gray neutral glass having a luminous transmission less than 20%, and preferably between 15 and 18% for a glass thickness between 3.9 and 4.1 mm., and a reduction on the solar energy transmission that is expressed as a transmission of infrared radiation less than 14%, and an ultraviolet radiation transmission less than 12%, is achieved.